The present invention relates to a method and apparatus in which compressed and purified air is cooled in a main heat exchanger and is then separated in a distillation column to produce a product stream that warms within the main heat exchanger. More particularly, the present invention relates to such a method and apparatus in which one or more of the product streams is distributed to a plurality of vortex tubes operated at successively warmer temperatures so that at least one of the vortex tubes produces a warm stream at a temperature warmer than the compressed and purified air upon its introduction into the main heat exchanger to generate refrigeration.
Air is separated by a variety of well-known cryogenic rectification processes. In such processes, the air is compressed and then purified to remove moisture, carbon dioxide and hydrocarbons. The compressed and purified air is then cooled within a main heat exchanger before being introduced into a distillation column system. The distillation column system can be a single column designed to produce nitrogen as a tower overhead or one in which the air is introduced into an intermediate location thereof so that an oxygen product is also produced as a column bottoms. Another common distillation column system is a double column having higher and lower pressure columns associated with one another in a heat transfer relationship. The higher pressure column produces nitrogen as a tower overhead which is in part condensed to reflux both the higher and lower pressure columns. The column bottoms produced in the higher pressure column is introduced into the lower pressure column for further refinement to produce an oxygen product.
Since all of the systems discussed above operate at cryogenic temperatures, the main heat exchanger and distillation column system must be insulated from the environment by an external structure known as a cold box. Nevertheless, there is heat leakage through the cold box and also warm end losses due to the temperature of the incoming air. As a result, any low temperature rectification process must be refrigerated in order for the process to remain in heat balance. To this end, refrigeration is generated by expansion machines such as turboexpanders. Either part of the incoming air or vaporized liquid bottoms or nitrogen may be heated and then expanded to a low temperature. Refrigeration is generated because energy is dissipated from the system as shaft work. For instance, the turboexpander can be connected to an electrical generator or an energy dissipative oil brake.
Turboexpanders and expansion machines, however, add expense and complexity to any air separation plant. As will be discussed, the present invention provides a method and apparatus for separating air does not use expansion machines such as turboexpanders and thus, produces a simpler and more cost effective plant than has been considered in the prior art.